1. Field of the Invention
The present invention relates to an objective lens for use in recording or reproducing an optical disk, such as a CD (compact disk), a DVD (digital video disk), etc., having a diffraction-limited performance; and to an optical device using such an objective lens, wherein CD includes recordable CD-R.
2. Discussion of the Background
There have been proposed objective lenses for reading DVD comprising a transparent substrate having a thickness of 0.6 mm and CD comprising a transparent substrate having a thickness of 1.2 mm (JP-A-10-255305, JP-A-11-16190, JP-A-11-2759).
In these conventional techniques, a phase shifter having a concave portion or a convex portion, which is formed in a ringed belt-like shape around the optical axis as the center, is provided in a surface of the objective lens having an aspheric surface. The basic structure of the objective lens is optimized so as to be capable of reading a DVD, preferably. When a CD is to be read, the phase is corrected by the phase shifter so that the aberration is made as small as possible. As a result, for both DVD and CD, the correction can preferably be achieved with respect to the on-axial aberration, in particular, the on-axial spherical aberration.
In the conventional techniques, however, a preferred correction could not be achieved with respect to the off-axial coma aberration at the time of reading a CD. Namely, since the off-axial coma aberration was large, the optical performance showed a large reduction when the light source, the objective lens, etc. was inclined from the optical axis or shifted from the optical axis. Therefore, high accuracy was required for positioning the objective lens and other elements in assembling the optical device. Hence, productivity was poor.
Further, since high accuracy was required in determining the position of the objective lens, gradual deterioration of the optical performance was caused with a lapse of time due to the wearing or another cause of a mechanism for moving the lens or the light source (a moving mechanism), the wearing resulting in an inclination or a shift of the objective lens from the optical axis.
Further, since the off-axial coma aberration was large, an allowable range became narrow in determining the position of the objective lens, in particular, a shift from the optical axis in autofocus driving to the objective lens when the optical device was actually operated. Therefore, there was the problem that the optical performance was reduced.
An object of the present invention is to solve the above-mentioned disadvantages, and to provide an objective lens capable of correcting well both the on-axial aberration and the off-axial aberration when a plurality of optical disks such as, for example, DVD, CD, etc. are recorded or read. A further objective is to provide an optical device using the objective lens.
In accordance with the present invention, there is provided an objective lens having aspheric surfaces at both surfaces, used for an optical system wherein light from one or two light sources having different wavelengths with respect to two optical disks is converged to respective data recording surfaces of the optical disks, and reflection light from the respective data recording surfaces is received by a light receiving element in recording or reading the optical disks, respectively. The objective lens is characterized in that in recording or reading a first optical disk, a first wavelength xcex1 is used, and in recording or reading a second optical disk, a second wavelength xcex2 is used. A first phase shifter provided with ring-like step portions having the center coincident with the optical axis is provided in a surface of the objective lens, the first phase shifter producing a phase difference with respect to light having the first wavelength. A second phase shifter provided with ring-like step portions having the center coincident with the optical axis is provided in the other surface of the objective lens, the second phase shifter producing a phase difference with respect to light having the second wavelength xcex2.
Further, there is provided an optical device comprising an objective lens having aspheric surfaces at both surfaces, wherein light from one or two light sources having different wavelengths with respect to two optical disks is converged to respective data recording surfaces of optical disks, and reflection light from the respective data recording surfaces is received by a light receiving element in recording or reading the optical disks, respectively.